Skin color surfaces are usually difficult in automatic white balance illuminant estimation. Firstly, skin tone could vary significantly from individuals to races; and secondly, it could be very close to and sometimes mingle with the white point locus under certain illuminations, which tends to illuminant estimation errors for some common illuminant estimation algorithms, such as A. The Grey World, B. Perfect Reflector and C. Color by Correlation Models. It is often found that even in top of the line professional or smartphone cameras automatic white balance errors and unpleasant skin tone rendition would occur, for example, by approaching the camera head to near skins. Therefore, in US patent application US2008037975 (A1), it is proposed to remove the detected face area as a noise from automatic white balance illuminant estimation.
Automatic white balance usually comprises steps of illuminant estimation and color gain adjustment followed by related color correction or enhancement. For illuminant estimation, it is a well-known classical phenomenon in automatic white balance that what comes into the camera lens is a mixed emission from surfaces with unknown reflectance spectrum properties illuminated by unknown ambient light sources. It would be much easier for automatic white balance if there is a reference color surface in the scene with known color properties such as a gray chart used for on-site white balance by professional photographers. However, that approach is cumbersome and not applicable to ordinary users for daily usage. Now it is observed that the face of a user of a Front-Side camera is often within the view field of the camera and comprises a considerable portion of the pictures taken, while back-side cameras and other consumer or surveillance digital cameras also take shots of human portraits more often than not. These findings and growing popularity of smartphones and other devices with Front-Side cameras make it a practical application for automatic white balance and related color rendition to use a reference surface within the image target itself, the properly characterized facial color features of the device user.